Motion responsive differential transformer transducer system



Nov. 19', 1968 M'LLAR 3,412,387

MOTION RESPONSIVE DIFFERENTIAL TRANSFORMER TRANSDUCER SYSTEM Filed Feb. 18, 1965 2 Sheets-Sheet 1 /7(//7 f/ .0. M/Y/Q/ INVENTOR.

ATTORNEVJ Nov. 19, 1968 H. D. MILLAR MOTION RESPONSIVE DIFFERENTIAL TRANSFORMER TRANSDUCER SYSTEM Filed Feb. 18, 1965 2 Sheets-Sheet 2.

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zm/z/fz A770 EVJ United States Patent 3,412,387 MOTION RESPONSIVE DIFFERENTIAL TRANS- FORMER TRANSDUCER SYSTEM Huntly D. Millar, Houston, Tex., assignor to E. & M.

Instrument Company, Inc., Houston, Tex., a corporation of Texas Filed Feb. 18, 1965, Ser. No. 433,735 9 Claims. (Cl. 340199) ABSTRACT OF THE DISCLOSURE A transducer utilizing a differential transformer with a movable armature wherein the armature is displaced a sufficient distance to avoid entering its null or centered position, and includes a partitioned Bourdon tube which may be connected to the movable armature when a fluid pressure system is connected to the transducer.

The present invention relates generally to a transducer. More particularly, the present invention relates to a transducer for converting a movement into an electrical signal.

Many types of transducers have been used for converting motion to an electrical signal. In such devices, it is essential that the electrical output be proportional to the degree of movement from the initial position.

Differential transformers have been used to convert this motion into an electrical output. It has been noted, however, that such differential transformers have a null or central position for their armature and that when the armature passes through this central position, the relationship of the electrical output to the position of the armature is not maintained in the desired direction proportion.

Various devices have been used to translate a sensed value, such as pressure into a motion, which motion is used to move the armature in a differential transformer. Bourdon tubes have been used for this purpose; however, difficulty has been encountered with Bourdon tubes when used with liquid systems in that it is extremely difiicut to completely eliminate all gas from a Bourdon tube.

Therefore, an object of the present invention is to provide a transducer converting a mechanical movement into an electrical output which is directly proportional to the position of a moving element throughout the whole range of movement of such element.

Another object of the present invention is to provide a pressure transducer including a differential transformer and a partitioned Bourdon tube connected to the armature of such transformer wherein the output of the transformer is direcly proportional to the positioning of the armature in such transformer by said Bourdon tube.

A further object of the present invention is to provide a differential transformer transducer in which the primary circuit of the transformer forms a part of the oscillation circuit to provide the energy for the differential transformer and also to provide the energy through a separate transformer for a direct current bucking voltage in the output circuit.

Another object of the present invention is to provide a differential transformer transducer in which the armature is offset from its central or null position and to provide a secondary bucking voltage from another transformer which is introduced in the output circuit of the differential transformer to balance the output resulting from the offset of the armature.

These and other objects of the present invention are hereinafter explained in relation to the drawings wherein:

FIGURE 1 is a view of the interior of a case containing a transducer constructed in accordance with the present invention.

3,412,387 Patented Nov. 19, 1968 "ice FIGURE 2 is a schematic wiring diagram of the primary and secondary circuits of the differential transformer of the present invention.

FIGURE 3 is a sectional view of the Bourdon tube taken along lines 33 in FIGURE 1.

FIGURE 4 is another sectional view of the Bourdon tube taken along lines 44 in FIGURE 1.

Referring more in detail to the drawings, the device of the present invention illustrated in FIGURE 1 is shown to be contained within'the case 10 which has the cover removed so that the individual components therein may readily be seen. The holes 11 are provided for fastening a cover (not shown) to the case. The circuit board 12 containing a substantial portion of circuitry necessary for the operation of the device of the present invention is secured to the case 10 by the screws 13. The plug 14 provides the external connection for the electrical circuits and extends through the case connecting to the wiring from the circuit board 12.

The device illustrated includes a Bourdon tube 15 which is mounted in a mounting member 16. The mounting member is secured to the case 10 by the set screw 17. Thus, the mounting member secures the one end of the Bourdon tube to which the lines 18 and 19 connect. The other end of the Bourdon tube 15 is free and substantially unrestricted in motion. A portion of the inner surface of the Bourdon tube is provided with the bimetallic member 20 secured thereto by cement or other suitable securing means to provide a temperature correction for the operation of the Bourdon tube 15. The free end of the Bourdon tube 15 is suitably secured to an arm 21 which is connected to the armature (not shown) of the differential transformer 22. Suitable leads 23 connect from the differential transformer 22 to the circuit board 12. The rod 24 is positioned adjacent the exterior of the differential transformer 22 and is secured in place by cementing or other suitable means. The function of the rod 24 is to provide an external influence on the fields generated in the transformer. The rod 25 which is adjustably positioned by rotation of the slotted head of pin 26 provides a further adjustment to the fields created by the transformer 22. 7

Referring to the wiring diagram of FIGURE 2, the differential transformer is designated generally with the number 27 and the movable armature with the number 28. The secondary or output side of the transformer 27 includes two coils which are connected in series opposed circuit whereby when the armature 28 is positioned in its null or central position, the output of the secondary coils will be offset against each other.

The connections 29 through 33 into the circuit illustrated in FIGURE 2 include a ground connection 29, the power connection 30 and the outlet connections 31, 32 and 33. The primary coil 34 of the transformer 27 forms a part of a simple transistor oscillator circuit which is used to excite the differential transformer 27. The primary winding 34 has two series condensers 35a and 35b across it to form a tuned circuit in the collector circuit of the transistor 36. The tap 37 between the condensers 35a and 35b is used to feed a signal to the emitter of the transistor 36. The resistance 38 connects from one end of the primary 34 to the base of the transistor 36. Resistance 39 connects from the base of the transistor 36 to the ground connection. Resistance 40 connects from the emitter of the transistor 36 to ground. Potential in relation to ground is provided across connection 30 and ground 29. Connection 30 extends to the upper side of the primary coil 34. The lower side of the primary coil 34 is connected to the collector of the transistor 36. The transformer T is provided with a primary coil 41 which induces a bucking voltage in the secondary coil 42 which is connected to the secondaries of the differential transformer 27, as hereinafter set forth. The primary coil 41 and capacitor 42a are in series and they are connected in parallel with the primary coil 34. Two test circuits are positioned in parallel across the primary 41. The first test circuit includes a switch 43 and the variable resistor 44. The second test circuit includes the switch 45, the resistor 46 and the variable resistor 47. As can be seen, actuation of the switch 43 will connect the circuit through the variable resistor 44 across the primary coil 41. Switches 43 and 45 should be spring loaded to remain in the position shown until actuated. When the switch 45 is actuated, it will connect the circuit through the switch 43, the switch 45, the resistor 46 and the variable resistor 47 and across the primary coil 41. These circuits are used for calibration and will function to reduce the bucking voltage by a predetermined amount which corresponds to a known input signal. A reduction in the bucking voltage produces the same effect as an increase in the output of the secondary coil.

As shown in FIGURE 2, the secondary coils 48 and 49 are connected in series opposed circuitry and have tuning condenser 50 connected across the lower leads from the coils 48 and 49. The diode 53 is positioned in the lower lead from the coil 48 to provide for rectifying the alternating current produced to a direct current. The resistor 51 and the condenser 52 are connected across the rectified output of transformer 27. Diode 56 is connected in the lower lead from the secondary coil 42. The secondary coil 42 of the bucking voltage circuit is provided with a resistor 54 and a capacitor 55 connected in parallel across the rectified output from secondary coil 42. The upper lead of secondary coil 42 is connected to the lower lead from the secondary 49. The variable resistor 57 is connected to the upper lead of condenser 52 and the lower lead of condenser 55 across the opposed rectified outputs from the transformers 27 and T. Capacitor 58 is positioned across the leads extending to connections 33 and 31.

From the foregoing it may be seen that the output circuitry is divided into two portions: the differential transformer output portion and the bucking voltage portion. The secondaries of the differential output transformer are connected in series opposed configuration so that when the movable armature 28 is symmetrically placed within the differential transformer 27, the output across the resonating condenser should be exactly zero. It has been found desirable to displace the movable armature 28 a sufficient distance whereby during all normal operations of the device of the present invention, the movable armature 28 will not pass through its normal zero or null region. By avoiding the movement of the armature through the null region, the following problems generally encountered in the differential transformer are overcome. In practice, a perfect zero or balance is difiicult to achieve because of resistive and capacitive unbalance in the transformer. The voltage output will be nonlinear with respect to the movement of the armature in the null region unless perfect electrical balance is achieved. The rectifying diode is a nonlinear device through its initial region of conduction. Single diode detection will provide an identical output whether the armature is displaced towards one secondary coil or towards the other; i.e., nondirectional detection.

Therefore, it is essential in this circuit configuration to initially displace the armature towards one of the secondary windings so that there is an initial direct current voltage across the output lead resistor. Provided the mechanical displacement of the armature is confined so that it does not traverse through the null region, the output voltage will increase or decrease in proportion to armature movement, with this increase or decrease being linear with respect to extent and direction of movement. It is preferable to have a zero voltage output which will fluctuate positively or negatively depending on armature movement. In this circuit an additional transformer has its primary 41 connected to the oscillator, and the rectified secondary voltage from the coil 42 is connected in series with reverse polarity to the secondaries of the differential transformer 27 so that the voltage across the load resistor 54 exactly balances the direct current voltage from the differential transformer circuit. The combined output circuitry provides a zero voltage output at the resting position of the armature and a positive or negative voltage directly proportional to the extent and direction of movement of the armature. The armature movement is restricted within the linear range of the differential transformer 27 on the one side and the null output position on the other.

The initial insertion of the armature is determined from the application. For instance, if the armature were linked to a Bourdon tube and the transducer were required to have a pressure range of 0-600 millimeters of mercury, the armature could be initially set close to the null point and would travel exclusively between that point and the outer limit of transducer linearity. If the transducer were required to operate between plus or minus 300 millimeters of mercury, the armature could be set initially in the mid position of linearity and travel for positive pressures towards the outer limit of linearity and for negative pressures towards the null position.

The bucking voltage is derived from the oscillator circuit and this makes the zero output of the combined output circuit relatively independent of the supply voltage to the transistor circuit.

With reference to the design of the Bourdon tube 15, as illustrated in FIGURES 1, 3 and 4, it should be noted that the tube normally will be hollow. In order to partition the Bourdon tube 15 whereby inlet and outlet connections may be made thereto for connection to the lines 18 and 19, both ends of the tube are left open while the partition 58 is inserted by any suitable method or as hereinafter more fully explained. The partition 58 then divides the tube 15 into two passages 59 and 60. When the partition 58 is completely installed within the Bourdon tube 15, a portion thereof is cut out to provide a communication between passage 59 and passage 60 near the closed end of the Bourdon tube 15. Thus, when the lines 18 and 19 are connected to the Bourdon tube 15 and the other end of the Bourdon tube is closed, the liquid in the system may be conducted through line 18 into passage 60 of the Bourdon tube 15. The liquid will flow to the end of the Bourdon tube and around the cut-out end of the partition 58 into passage 59 and out through line 19. In this way any liquid which is to be measured by the device of the present invention will completely fill all of the Bourdon tube. Thus, any inaccuracies which might result from having gas trapped within the closed end of the Bourdon tube are eliminated from the system.

As pressure of the liquid in the system being measured increases, the increase in pressure will cause the Bourdon tube 15 to attempt to straighten and will cause the closed end of the Bourdon tube, as seen in FIGURE 1, to move downwardly thereby displacing the armature in the differential transformer 22 downwardly to provide an electric output of the secondary circuits directly proportional to the pressure of the liquid.

As hereinbefore mentioned, the partitioning of the Bourdon tube may be accomplished in any suitable manner; however, it is suggested that it may be done as hereinafter described. A standard Bourdon tube to be used in such applications should be open at both ends. Two plastic tubular members each taking up approximately the areas of the passages 59 and 60, as illustrated in FIG- URES 3 and 4, should be passed lengthwise through the tube until the ends of the members extend from each end of the Bourdon tube. With the inserted members spaced apart throughout their length, a suitable cement or plastic, which, when set, will adhere to the Bourdon tube but will not adhere to the inserted tubes, is pumped through the space between the inserted tubes in the Bourdon tube. This plastic material is allowed to set and when set,

the inserted members are removed. The excess material is trimmed from the one end, as hereinbefore described, to provide the cross passage between passage 59 and passage 60 at the closed end of the Bourdon tube. The Bourdon tube is then closed at such end and the other end is provided with suitable connections into the passages 59 and 60.

Thus, with the Bourdon tube partitioned as hereinbefore defined, inaccuracies which would result in a liquid pressure measuring system from having a compressible gas in the system are completely eliminated. The fluid passage through the Bourdon tube is continuous allowing a complete filling of the tube with the fluid in the system being measured. The travel of the tube at its closed end will not be affected if the material used for the partition is a relatively flexible material. With the partition in the Bourdon tube it has been found that movement of the closed end of the tube will be linearly proportional to the pressure applied to the interior of the tube. As used in the present application, such a Bourdon tube will actually position the armature of a differential transformer in a position in which the amount of displacement of the armature will be directly related to the pressure within the Bourdon tube.

From the foregoing it can be seen that the present invention provides an improved transducer having an electrical output directly proportional to the input to the device. The present invention further provides a pressure tranducer including a differential transformer and a partitioned Bourdon tube for use in measuring fluid pressures, and the electric output of such transducer is directly proportional to the pressure in the system being measured.

The foregoing disclosure and description of the invention are illustrative and explanatory thereof and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made within the scope of the appended claims without departing from the spirit of the invention.

What is claimed is:

1. A differential transformer transducer comprising:

a primary winding,

secondary windings connected in series opposed circuit,

an armature,.

means moving said armature with respect to said windings,

a transistor,

two series condensers connected across said primary winding to form a tuned circuit in the collector circuit of said transistor,

means connecting the emitter of said transistor to a point between said two condensers,

means connecting the base of said transistor to a biasing potential,

means providing potential to said primary winding, and

output means connected to said secondary winding.

2. A pressure transducer comprising:

a differential transformer having a primary winding,

secondary windings connected in series opposed circuit and amovable armature,

a Bourdon tube having a closed end and an open end,

said tube being partitioned lengthwise,

a passageway through said partition at the closed end of said tube,

means connecting the open end of said tube to a fluid pressure system,

means connecting the closed end of said tube to said 1 a differential transformer having a primary winding,

secondary'windings connected in series opposed circuit and a movable armature,

a, second transformer having a primary winding and a secondary winding, and

an oscillating circuit connected to excite said primary windings of said differential transformer and said second transformer,

said movable armature being at all times offset in one direction from its null position within said differential transformer,

said secondary winding of said second transformer being connected in series with said circuit of said secondary windings of said differential transformer whereby the rectified voltage outputs are in series opposition and will be directly proportional to the position of said movable armature within said differential transformer.

. A transducer according to claim 3 including:

test circuit incluling a switch and a resistor across the primary of said second transformer whereby closing of said switch will change the voltage output in said circuit by a predetermined amount. 5. A transducer according to claim 4 wherein: said resistor is a variable resistor whereby said valtage output in said circuit is varied in accordance with the effective resistance of said variable resistor. 6. A transducer according to claim 3 including: a pair of test circuits positioned in parallel across the primary of said second transformer, each of said test circuits including a switch and a resistor, connection of one of said test circuits by the closing of its switch changing the voltage output in said circuit of the secondary windings by a predetermined amount and connection of the other of said test circuits by the closing of its switch changing the voltage output in said circuit of the secondary windings by a different amount.

. A transducer comprising:

differential transformer having a primary winding, secondary windings connected in series opposed circuit and a movable armature,

second transformer having a primary winding and a secondary winding,

means energizing the primary windings of said transformers from a common source, and

means for moving said movable armature within said differential transformer,

said means moving said armature having an at-rest position offsetting said armature from its null position within said differential transformer,

said secondary winding of said second transformer being connected in series and with reverse polarity with said secondary windings circuit whereby the output of said combined secondary is zero when said moving means for said armature is in its at-rest position.

8. As a subcombination in a transducer having a movable means, a pressure responsive device, comprising:

a Bourdon tube having a closed end and an open end,

and

means partitioning said tube lengthwise,

said partition means defining a passageway between opposite sides of said partition means at the closed end of said tube,

the open end of said tube being adapted to be connected to a fluid pressure system,

the closed end of said tube being adapted to be connected to the movable means of the transducer whereby said movable means is moved by the closed end of said tube responsive to fluid pressure in said tube.

9. A transducer comprising:

a differential transformer having a primary winding,

secondary windings connected in series opposed circult and a movable armature,

a second transformer having a primary winding and a secondary winding, and

circuit means connected to excite said primary windings of said differential transformer and said second transformer,

said movable armature being at all times offset in one direction from its null position within said differential transformer,

said secondary winding of said second transformer being connected in series with said circuit of said secondary winding of said differential transformer whereby rectified voltage outputs are in series opposition and will be directly proportional to the position of said movable armature within said differential transformer.

References Cited UNITED STATES PATENTS Caldwell 73-418 Nilson 340199 X Macgeorge 340199 X Motherwell 73411 Macgeorge 340-196 Light 321-2 Macgeorge 340199 Bender 321-2 Collins 323-51 MILTON O. HIRSHFIELD, Primary Examiner. 5 W. E. RAY, Assistant Examiner. 

